1. Field of the Invention
The present invention relates in general to an apparatus for controlling a drive force to drive a motor vehicle, and more particularly, to techniques for improving the running stability and drivability of the vehicle, irrespective of a variation in the running load of the vehicle, over the entire ranges of the engine load and the vehicle speed.
2. Discussion of the Related Art
For controlling a drive force to drive a motor vehicle, there is known a drive force control apparatus adapted to change the vehicle drive force by adjusting the opening angle of a throttle valve of a vehicle engine or by controlling the speed ratio of an automatic transmission of the vehicle. For example, a motor vehicle equipped with an automatic transmission which is automatically shifted to change the speed ratio is provided with a shift control apparatus adapted to change the speed ratio of the transmission on the basis of the actually detected engine load and vehicle speed and according to a predetermined shift pattern. The shift pattern usually consists of shift-up and shift-down boundary lines each representing a predetermined relationship between the engine load and the vehicle speed. The appropriate shift-up or shift-down boundary line is selected depending upon the currently selected position of the transmission, and the transmission is shifted up or down according to the selected shift-up or shift-down boundary line depending upon the detected engine load and vehicle speed, whereby the vehicle drive force is suitably controlled. The shift-up and shift-down boundary lines used by such transmission shift control apparatus are generally formulated to achieve not only high running stability and drivability of the vehicle but also high fuel economy of the vehicle, when the vehicle is running on a generally level or flat road surface, since the vehicle usually runs on such flat road surface. Accordingly, the drive force tends to be insufficient for the intended drivability of the vehicle when the vehicle runs uphill.
In the light of the above-indicated insufficiency of the drive force during uphill running of the vehicle, a transmission shift control apparatus is proposed as disclosed in JP-A-62-180153 (published in 1987), wherein an uphill running of the vehicle is detected if the vehicle speed is lowered across the selected shift-up boundary line while the throttle opening angle is larger than a predetermined value. When the uphill running is detected by the shift control apparatus, a shift-up action of the automatic transmission is inhibited, and the transmission is held in the currently established gear position, whereby the vehicle is driven with a relatively large drive force for improved running stability and drivability during uphill running of the vehicle.
However, the known transmission shift control apparatus indicated above is adapted to detect the uphill running of the vehicle under a limited running condition of the vehicle, namely, only if the vehicle speed is lowered across the selected shift-down boundary line and if the throttle opening angle is larger than the predetermined lower limit. This arrangement does not provide an improvement in the control of the vehicle drive force depending upon a variation in the load acting on the vehicle, when the throttle opening angle is smaller than the predetermined lower limit. In other words, the known arrangement does not permit satisfactory running stability and drivability of the vehicle over the entire ranges of the engine load (throttle opening angle) and the vehicle speed.
There is also known a transmission shift control apparatus as disclosed in JP-A-3-206326 (published in 1991), in which a shift-down vehicle speed as obtained from a shift-down boundary line is adjusted on the basis of the operating speed of the accelerator pedal or a rate of change .DELTA.TA of the opening angle TA of the throttle valve, so that the transmission is likely to be shifted down when the accelerator pedal is operated at a relatively high speed, that is, when the vehicle driver has a relatively strong desire to accelerate the vehicle. According to this arrangement, the shift-down vehicle speed as determined by the appropriate shift-down boundary line is suitably adjusted depending upon the desire of the vehicle driver to accelerate the vehicle, so that the transmission is more likely to be shifted down when the accelerator pedal is depressed relatively rapidly, than when the same is depressed relatively slowly. Thus, the vehicle driver's desire to accelerate the vehicle is reflected by the shifting actions of the automatic transmission, for improved drivability of the vehicle.
However, the adjustment of the shift-down vehicle speed in the shift control apparatus described just above results in reducing a difference between the shift-down vehicle speed and the shift-up vehicle speed. Although this does not cause a problem when the altitude of the vehicle is relatively low as in normal running of the vehicle, the reduced difference may cause frequent shift-down and shift-up actions of the transmission when the altitude of the vehicle is relatively high, because of lowered output of the engine at such high altitude. Described more specifically, such frequent shift-down and shift-up actions of the transmission would occur due to the behaviors of the vehicle driver and the known shift control apparatus during high-altitude running of the vehicle. That is, a shift-down action of the transmission causes an increase in the vehicle drive force and an increase in the vehicle speed, resulting in an automatic shift-up action of the transmission, which would motivate the vehicle driver to depress the accelerator pedal in an attempt to maintain the vehicle speed. Consequently, the transmission is shifted down again. Thus, the transmission tends to be shifted up and down frequently during running of the vehicle at a relatively high altitude, if the shift-down vehicle speed is adjusted as described above.